Semiconductor lasers are frequently used as laser light sources in SMI laser sensors. If these lasers are operated with a defined current shape, for example, a periodic sawtooth or triangular current, the output frequency of the laser almost instantaneously follows these current variations due to the simultaneously changed optical resonator length. The resulting difference in frequency between the resonating and the back-scattered light can be evaluated in a suitable evaluation unit and translated back to the desired position or velocity information.
Infrared vertical cavity surface-emitting lasers (VCSEL) are quite common in optical communication applications. The laser cavity consists of two stacks of Distributed Bragg Reflectors (DBRs), which are epitaxially grown on a suitable substrate and enclose a gain region made up of several quantum wells. The DBR layers also take over the task of feeding current into the gain region. Therefore, one of the DBRs is usually n-doped and the other p-doped. In such a VCSEL, one of the DBRs is designed to be highly reflective, typically the p-DBR with a reflectivity of >99.9% for the lasing wavelengths, while the other one allows efficient outcoupling of the laser radiation and thus also feedback from the target object into the laser cavity. VCSELs have the great advantage that their surface-emitting properties render them suitable for production and testing on a wafer level in large quantities, which opens the possibility of a low-cost production process. Furthermore, the output power can be scaled to a certain extent via the area of the emitting surface. Larger output powers can be achieved by using VCSEL arrays.
A known laser sensor based on self-mixing interferometry and comprising a VCSEL as the laser light source is the Laser Beetle of Philips. This laser sensor is used as a motion sensor in PC laser mice. The VCSEL emits infrared light around 1 μm wavelength with a typical output power of a few milliwatts. Due to the short coherence length of the VCSEL and the low output power density, the detection range of this sensor is limited to several millimeters. This short detection range is a main drawback of laser sensors with VCSELs as laser light sources. Since the sensing principle of an SMI laser sensor makes use of the interference between the resonating and back-scattered radiation, the maximally accessible range of such a device is limited to half the coherence length lc of the laser radiation, which is approximated by
            I      c        ≈                  λ        0        2                    Δλ        FWHM              ,
wherein λ0 is the center wavelength of the laser radiation and ΔλFWHM is its spectral line width (full width at half maximum). Estimating the coherence length of existing VCSEL-based SMI sensors from a typical center wavelength of ˜1 μm and an emission half width of ˜0.2 nm, a value of lc≈5 mm is obtained, which is well in line with the detection range of existing devices. However, the short detection range limits the application of such VCSEL-based SMI laser sensors to short-range applications.